COOLING PRIZE PAPER
Method for cleaning and checking the
base of diaphragm wall panels
AG Berry, Arup, UK
Base cleaning is common for bearing piles, allowing the end-bearing
capacity of the bored pile to be
included in the design. However,
the base cleaning of diaphragm wall
panels is not common in the UK.
This paper describes the author’s
experience of a diaphragm panel
wall base cleaning process that
was developed for a load-bearing
retaining wall for a large basement
in east London. The extra vertical
capacity achieved by the clean base
allowed end-bearing to be taken
into account in addition to skin friction, providing substantial savings
in the number of foundation piles
required.
The base-cleaning process developed consists of two stages. The first
comprises scraping away ridges in the
panel base left behind by the clamshell
excavation bucket. Successful removal of the ridges is verified by means
of a weight attached to a measuring
tape wide enough to sit on top of
any ridges.
The second stage involves removing soft sludge from the panel base
using a submersible pump placed at
the base of the excavation. This is
combined with the process of replacing the dirty support fluid in the
panel with clean fluid to minimise
the risk of suspended soil settling on
to the base. The panel base cleanliness is verified by checking the hardness of the panel base using a qualitative scale to assess the impact felt
when a standard weight is lowered
on to the base of the panel.
Introduction
Base cleaning of bearing piles for
end bearing in London clay, and as
a preparation for base grouting in
Thanet sand, is a common practice
in London. However, base cleaning
and base grouting is less common
for retaining walls as is use of these
structures for load bearing.
During construction of a 600m
perimeter diaphragm wall for a
15m-deep basement in east London,
base grouting trials were performed
on a number of diaphragm wall
panels. Although the grouting trials
were inconclusive because of leakage problems in the injection system,
the base-cleaning method developed
was adopted for all the load-bearing
panels. The base-cleaning method
was developed by Arup in conjunction with the contractor, a Costain/
Bachy Soletanche joint venture.
GROUND ENGINEERING NOVEMBER
2009
LOAD
Abstract
Qs+Qb
(Qs+Qb)/2.5 or Qs/1.5
Base cleaning
Qs/2.5
Good base
Poor base
Shaft only
DISPLACEMENT
Figure 1: Illustration of how base cleaning allows greater design loads
Project background
The site stratigraphy is given in
Table 1 (below). The load bearing
panels were founded in Thanet
sand with toe levels generally
varying between -17.5mOD and
-26.0mOD. Thanet sand on the
site comprises a very dense, greengrey, silty-fine sand, with the silt
content increasing below approximately -22.5mOD. Thanet sand
is commonly used as a founding
medium for bored piles in this area
of London.
During design, a factor of safety
is usually applied to the combined
shaft capacity Qs and base capacity
Qb, and a smaller factor of safety
is applied to shaft capacity alone
to allow for a serviceability check.
This allows deflections at working
load to be limited.
Figure 1 (above) shows the loaddeflection curves for a deep foundaStratum
Made ground
tion with a cleaned base and poorquality base. It can be seen that if
the base quality is not controlled
then the load deflection behaviour
of the foundation can be very variable, preventing reliance on Qb in
design and requiring a larger factor
of safety to be applied to Qs alone.
The diaphragm wall panels
were designed making reference
to Eurocode 7 (British Standards
Institution 2004) and BS8004
(British Standards Institution 1986)
in consultation with the local
building control officer.
In the case of this project,
providing a clean, reliable panel
base allowed the design to be based
on the lower capacity from a global
factor of safety of 2.5 applied to (Qs
+ Qb), or 1.5 applied to Qs alone.
If the panel bases had not been
cleaned then design would have
been required to be based on a factor
Top of stratum mOD
+5.3
Alluvium
0.0
Terrace gravel
-2.0
Clayey Lambeth group
-6.0
Sandy Lambeth group
-11.0
Thanet sand
-17.0
Chalk
-31.0
Table 1: Site stratigraphy
of safety of 2.5 applied to Qs alone.
The additional capacity available
from base cleaning was equivalent
to approximately two-thirds of
the vertical load capacity of a
continuous flight auger (CFA)
pile on the same site. Cleaning
the panel base to increase vertical
load capacity had a similar cost to
constructing equivalent CFA pile
capacity, but provided materials
savings.
The
diaphragm
wall
was
constructed in accordance with
ICE SPERW (Institution of Civil
Engineers 1996) and BS EN
1538:2000 (British Standards Institution 2000). The diaphragm wall
panels were excavated under bentonite using a toothed clamshell
grab. The base cleaning for the
diaphragm wall panels consisted of
two processes:
l removal of ridges left in the panel
base by the grab
l removal of sludge from the panel
base and prevention of further
sludge accumulating.
Ridge removal
1. Ridges in the panel base
Toothed clamshell grabs leave ridges
in a panel base that could break off
during concreting or collect soft
material that settles out from the
support fluid. Figure 2 (see overleaf)
illustrates ridges left by the grab
in made ground during a trial to
ascertain their probable size and
nature. The made ground at the
site had a typical corrected SPT of
0 to 10 blows per 300mm and so is
considerably softer than the Thanet
sand (corrected SPT typically 50
to 100 blows per 300mm). It was
therefore considered that the depth
of the ridges observed in this trial
would represent an upper bound.
The depths of the ridges were
measured and it was found that
except at the very edge of the bite
their depth was similar to the distance
between the tooth and the main
bucket on a closed grab (see figure 3,
overleaf). Measurement of the ridges
left behind and the tooth width
on the grab showed that the ridges
were approximately 100mm wide. It
can also be seen that the top of the
ridges form a curved shape towards
the edge of the panel. However,
it should be noted that even in the
relatively weak made ground the
shape of the “panel base” is
29
COOLING PRIZE PAPER
flatter than the circular arc
traced by the grab when it is closed
in mid-air.
Figure 2: Ridges left by the grab in made ground
Figure 3: Distance between tooth and main grab
≈ maximum ridge height.
Figure 4: Cleaning grabs: with scraper plates attached (left) and
with teeth removed (right)
30
2. Ridge-removal technique
A cleaning grab was developed with
the contractor to scrape away these
ridges and provide a flat base. The
scraper initially developed consisted
of stiffened flat plates that attached to
the tooth connections of a standard
clamshell rope grab, as illustrated in
Figure 4. These plates were effective
in removing the ridges, but progress
was slow as the plates prevented the
grab from closing fully and time was
lost re-digging material dropped
from the grab.
As an alternative to the scraper
plates the contractor decided to use
a standard rope grab with the teeth
cut off (see figure 4). This grab could
close completely, allowing much
faster base cleaning. In addition,
attaching a dedicated scraper grab
saved time compared with attaching
plates to a standard grab.
Figure 5 shows spoil scraped
up by the toothless cleaning grab,
which contains elongated chunks of
material. Inspection of this material
revealed it to be dense Thanet
sand, providing confidence that the
scraper grab was indeed removing
the ridges as well as scooping up
any loose material that had fallen to
the base.
3. Ridge removal verification
The trials of the scraper plates in
made ground and observation of
the scraped spoil in the Thanet sand
gave confidence that the method
was capable of removing base
ridges in Thanet sand. However, it
was necessary to develop a reliable
method of verifying, under up to
31m head of bentonite, that in every
base cleaned panel the ridges had
been effectively removed. This was
done by means of a weight welded
to a stiff wire mesh, approximately
0.3m2, and attached to the end
of a measuring tape, as shown in
figure 6. The width of the mesh
caused the weight to sit on the top
of the ridges, thus the depth to the
shallowest part of the panel was
measured.
Once the ridges had been scraped
off then the weight was able to sit
on the now-flat panel base, giving an
increase in the measured depth. Base
scraping was deemed to be complete
when the depth, as measured with
the “meshed” weight, had increased
by the initial height of the ridges (as
ascertained by measurement of the
grab geometry and during the trials
in made ground).
Each panel base was measured in
at least five locations – at each edge
of the panel, in the middle of the
side bites and in the centre bite. Care
was taken not to drag the weight
sideways, but rather to lower it in
from the top of the panel to provide
more certainty as to which part of
the base was being checked. The
amount of material removed by the
grab was also checked visually.
Sludge removal and prevention
1. Sludge removal technique
Once the base ridges had been
satisfactorily scraped away it was
necessary to remove any loose
sediment from the base of the panel
and prevent further settlement of
debris.
This was achieved by replacing the bentonite in the panel with
“fresh” bentonite by means of a
powerful submersible “Toyo” pump
placed at the base of the panel. This
pump sucked up the dense digging
bentonite and removed loose sediment from the base, while fresh bentonite was added at the top of the
panel.
Once tests indicated that the sand
content of the bentonite extracted
from the base of the panel was less
than 2% the pump was moved to a
new part of the excavation and the
process repeated.
Figure 5: Spoil from the toothless grab. Ridge-shaped chunks of
material have been outlined
GROUND ENGINEERING NOVEMBER
2009
2. Sludge removal verification
Verification of successful sludge
removal was achieved by sand
content testing of the bentonite
and also base hardness testing,
using a qualitative, but repeatable,
method developed by Arup for
measuring the base hardness during
construction of base grouted piles.
The method involves lowering a
heavy square weight on a tape, as
shown in Figure 7, to the base and
judging the hardness of the base
depending on the impact felt when
the weight touches the bottom.
The base is graded on a scale
of 1 to 5, as described in Table 2
(see below). A hardness of 1 to 3
is considered acceptable. While
not a quantitative measurement,
assessment of base hardness by
this method is easy to learn and
has been shown to relate well to
other indicators of base quality,
such as ease of base grout injection,
presence of soft clayey material on
the weight when it is raised to the
surface and observations of cores
taken through the panel base.
Base hardness was tested in the
same locations as the ridge removal
verification and the panel depth was
also checked for any rise in level that
might indicate contamination of the
base after the end of the scraping
process. If a raise in base level was
observed the panel was re-scraped.
Where the base was not hard
enough the Toyo pump was placed
over the soft area for a further 5 to
10 minutes, after which the hardness
was checked again. Generally
this extra 10 minutes of pumping
increased the hardness of the soft
area from 4 to 2 due to remaining
sludge being sucked away.
REFERENCES
n British Standards Institution.
1986. BS 8004:1986 Code
of practice for foundations.
British Standards Institution.
n British Standards
Institution. 2000. BS EN
1538:2000 Execution of
special geotechnical works
– diaphragm walls. British
Standards Institution.
n British Standards Institution.
2004. BS EN 1997-1:2004
Eurocode 7: Geotechnical
design - part 1: general rules.
British Standards Institution.
n Institution of Civil Engineers
in collaboration with the
Highways Agency, Ove Arup &
Partners, and the Federation
of Piling Specialists. 1996.
Specification for Piling and
Embedded Retaining Walls.
London, Thomas Telford.
GROUND ENGINEERING NOVEMBER
2009
Figure 6: Meshed weight to sit on top of base ridges.
Figure 8: 30mm of clayey
material found between the
concrete and the Thanet Sand
where a soft base was detected
Figure 7: Base hardness weight and tape
Because of the relatively large
volume of a diaphragm wall panel
and restrictions on site working
hours it was not always possible to
complete the entire base scraping,
bentonite replacement and sludge
removal, reinforcement placement
and concreting all in one day. In
such cases the cage placement was
delayed until the day of the pour
where possible as remediation of
a base once the cage was in place
would have been difficult.
On occasions when a cage
was left in a base cleaned panel
overnight ready for concreting
early the next morning the clean
bentonite was left re-circulating in
the panel. Bentonite was sucked
from the top of excavation and
returned to the panel base via tremie
pipes so the net flow of bentonite
would be upwards, counteracting
settlement of suspended material.
The sand content of the bentonite at
the bottom of the panel and the base
hardness and depth were checked
immediately prior to concreting.
Reliability of the
cleanliness checks
On three out of 51 load-bearing panels, circumstances dictated that the
panel was concreted despite a substandard base having been indicated
by the checks. This was undesirable
as it required a re-assessment of the
panel capacity, however, these occasions did provide a chance to assess
the reliability of the base cleanliness
verification method.
Figure 8 shows a core taken
through the base of a panel where an
unacceptable base hardness of 4 had
been detected; 20-30mm of clayey
material can be seen between the
concrete and the Thanet sand. Figure
9 shows a core taken through the
base of a panel where a base level rise
of 200mm was detected. In this core
there was approximately 200mm of
foreign material found between the
concrete and Thanet sand.
Other cores were taken where
no unusual depth changes were
detected and the base hardness prior
to concreting was in the range of
Grade
Base
quality
Observation by engineer with weighted tape
1
Hard
Sharp impact
2
Firm
Sudden impact felt, but some damping of
impact vibrations
3
OK
Distinct base detected with slight embedding of
weight into soil
4
Soft
Significant embedding of weight making definitive
base difficult to detect
5
Very soft
Weight sinks slowly into soil, making definitive base
very difficult to detect
Table 2. Arup base quality grading scale
Figure 9: 200mm of coarse sand,
clay and ceramic fragments
found between the concrete and
Thanet sand where a rise in base
level was detected
1 to 3 showed an acceptable contact
between the concrete and the
Thanet sand. This indicates that
the verification procedure developed
at this site to check the base was
clean is sufficient to detect an
unacceptable base.
Conclusion
A method has been developed for
cleaning and checking the base
of diaphragm wall panels, which
allows the design vertical capacity
of the wall to be increased when
compared with a standard diaphragm wall panel. This method
comprises two stages: creating a flat
base by scraping away ridges, and
preventing material from accumulating on the base. A wide weight
attached to a measuring tape can be
used to check successful removal of
base ridges and a square “hardness”
weight attached to a tape allows the
degree of soft material on the base
to be assessed.
31